1. Field of the Invention
This invention relates to a focal plane shutter for digital cameras.
2. Description of Related Art
In focal plane shutters for digital cameras, like focal plane shutters for silver-halide film cameras, there are types in which two shutters, a first blade and a second blade, are provided. The first blade and the second blade are actuated successively in the same direction in photography so that the imaging surface of an image sensor, such as a CCD, is continuously exposed from one end to the other. These first and second blades are connected to a driving member for the first blade and a driving member for the second blade, and after the release of a camera, power to an electromagnet for the first blade and an electromagnet for the second blade is disconnected in turn so that the above driving members are rotated successively by biasing forces of their driving springs and thereby an exposure operation is performed.
The first blade and the second blade are such that after the exposure operation is completed, a set member is actuated from its initial position to rotate the driving members against the biasing forces of the driving springs and thereby the first blade and the second blade are actuated toward their set positions. The set member remains at the set position without returning to the initial position until the release button of the camera is pushed in the next photography. After the release button of the camera is pushed in the next photography and the driving members are securely retained by attractive forces of the electromagnets, the set member is restored to the initial position. In a digital camera equipped with a focal plane shutter of this type, therefore, the imaging surface of the image sensor is covered by the first blade in a set condition, and thus an optical finder, as in the case of the silver-halide film camera, becomes necessary for a finder observing an image of an object to be photographed.
However, when the shutter is constructed so that, in the set operation by the set member mentioned above, the second blade is actuated toward the set position, while the first blade is restrained at the position where the exposure operation is completed, it becomes possible to cause light from the object to strike the imaging surface of the image sensor in the set condition. Hence, the image of the object formed on the imaging surface can be displayed and observed on a monitor such as a liquid crystal display device. However, in the case where the shutter is constructed as mentioned above, it is necessary that when the release button is pushed in photography, the restraint of the first blade is first released so that the first blade is actuated toward the starting position of the exposure operation and after the imaging surface of the image sensor is once brought into a covered state, the first blade and the second blade start the exposure operation as described above.
An example of a focal plane shutter for digital cameras constructed as mentioned above is disclosed in Japanese Patent Kokai No. 2001-222059. According to the structure of this shutter, a conventional driving member for the first blade is designed to include a first driving member for the first blade connecting the first blade and a second driving member for the first blade rotated by the driving spring in the exposure operation. In the set operation, the set member keeps the restraint of the first driving member for the first blade in the proximity of the position where the exposure operation is completed and moves only the second driving member for the first blade toward the set position. After that, when the release button is pushed and the set member is restored to the initial position, the set member releases its restraint so that the first driving member for the first blade and the first blade can be actuated toward starting positions of the exposure operation by biasing forces of set springs.
The focal plane shutter set forth in Kokai No. 2001-222059, however, is constructed so that when the set member is restored to the initial position after the release button of the camera is pushed, the restraint of the first driving member for the first blade is released immediately before the set member reaches the initial position, and the first blade is actuated from the proximity of the position where the exposure operation is completed toward the starting position of the exposure operation. Consequently, the first driving member for the first blade and the first blade, after bounding at the starting positions of the exposure operation, are brought into fixed states after the set member is restored to the initial position. Hence, after the set member is restored to the initial position, the exposure operation of the first blade cannot be immediately started and after the release button is pushed, time required until the first blade starts the exposure operation becomes long. This causes the problem that when the object is moving, there is a high possibility of missing a shutter chance.
Thus, a special restraining member is rotatably mounted on the same shaft as the set member so that when the set operation is performed, the restraining member, instead of the set member, restrains the first driving member for the first blade immediately in the proximity of the position where the exposure operation is completed. When the release button is pushed and the set member is restored to the initial position, the restraint is released at the initial stage of this restoration operation, while when the set member has been restored to the initial position, the first driving member for the first blade and the first blade, after bounding at the starting positions of the exposure operation, are brought into the fixed states. A focal plane shutter for digital cameras constructed in this way is disclosed in Japanese Patent Kokai No. 2003-222928.
In the structure of the focal plane shutter set forth in Kokai No. 2003-222928, in addition to the fact that the set member and the restraining member are mounted on the same shaft, as mentioned above, the set member is provided with a hook section that has flexibility and is configured with an engaging part at its top, and the restraining member is configured with two parts to be engaged on the periphery. When the set operation is performed by the set member, the engaging part of the hook section engages one part to be engaged and the restraining member is rotated in such a way that it is pulled. After the restraining member abuts on a stopper at the position where it is capable of restraining the rotation of the first driving member for the first blade, its engagement is released by the flexure of the hook section and only the set member continues to rotate so that, at the set position, the hook section is restored to the original by its own resilience and then is stopped. When the operation is carried out so that the set member is restored to the original position, the engaging part of the hook section engages the other part to be engaged of the restraining member to rotate the restraining member in such a way as to push it, and thereby the restraint of the first driving member for the first blade is released. After that, when the restraining member abuts on the stopper, its engagement is released by the flexure of the hook section, and immediately before reaching the set position, the hook section is restored to the original by its own resilience and then is stopped.
The focal plane shutter described in Kokai No. 2003-222928 is constructed so that the set member reciprocates and rotates the restraining member through the engagement of the engaging part of the hook section with two parts to be engaged of the restraining member and this engagement is released by the flexure of the hook section. However, it is extremely difficult to actually mass-produce the focal plane shutter constructed in this way. This is because it is very difficult to fabricate the shutters so that the resilience of the hook section is uniformly provided or so that the friction surface of the engaging part and those of the two parts to be engaged have a preset arrangement relationship.
Consequently, in the case where such focal plane shutters are mass-produced, even though the restraining member can be rotated by the engagement of the engaging part with the parts to be engaged, sometimes the hook section is not favorably flexed and the engagement of the engaging part with the parts to be engaged is not smoothly released when the rotation of the restraining member is stopped. Further, even though the engagement of the engaging part with the parts to be engaged can be smoothly released when the rotation of the restraining member is stopped, the engagement of the engaging part with the parts to be engaged may be released before the restraining member, when rotated, abuts on the stopper at a preset position of rotation. In particular, when the set member is restored to the initial position, the part to be restrained of the first driving member for the first blade is pressed against the restraining part of the restraining member by the biasing force of the set spring, and thus before the restraint of the first driving member for the first blade is released the hook section may be flexed to release the engagement of the engaging part with the parts to be engaged. Moreover, cases sometimes occur in which, while the set member is repeatedly rotated, the hook section, after being flexed, ceases to be restored to a preset original shape, the engagement of the engaging part with the parts to be engaged is not favorably carried out, and the restraining member cannot be securely rotated by the set member.